Natural gas is the most important fuel gas in the United States and provides more than one-fifth of all the primary energy used in the United States, Natural gas is also used extensively as a basic raw material in the petrochemical and other chemical process industries. The composition of natural gas varies widely from field to field. For example, a raw gas stream may contain as much as 95% methane, with only minor amounts of other hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide, or water vapor. On the other hand, streams that contain relatively large proportions of heavier hydrocarbons and/or other contaminants are common. Before the raw gas can be sent to the supply pipeline, it must usually be treated to remove at least one of these contaminants.
As it travels from the wellhead to the processing plant and, ultimately, to the supply pipeline, gas may pass through compressors or other field equipment. These units require power, and it is desirable to run them using gas engines fired by natural gas from the field. Since the gas has not yet been brought to specification, however, this practice may expose the engine to fuel that is of overly high BTU value, low methane number, or corrosive.
In the gas processing plant itself, heavy hydrocarbons are often removed by condensation. Such a method is impractical in the field, however, because sources of external cooling or refrigeration are not available. Furthermore, cooling of the raw gas, which still contains substantial quantities of water vapor, is likely to bring the gas to a pressure temperature composition condition under which hydrates can begin to crystallize, thereby clogging the condensation equipment and preventing gas flow.
That membranes can separate C2+ hydrocarbons from gas mixtures, such as natural gas, is known, for example, from U.S. Pat. Nos. 4,857,078; 5,281,255; and 5,501,722. Separation of acid gases from other gases is taught, for example, in U.S. Pat. No. 4,963,165. It has also been recognized that condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843; and 5,374,300.
It is also known to operate membrane systems at reduced temperatures by deliberately cooling the incoming feed stream, as is taught in U.S. Pat. No. 5,352,272, and to use the Joule-Thomson cooling produced by transmembrane permeation to facilitate upstream condensation, as is taught in U.S. Pat. No. 5,762,635.
U.S. PG Pub. No. 2004/0168570, to Franek, discloses an apparatus and process for separating purified methane from hydrocarbons higher than C1 in a feed gas mixture using a membrane. The membrane has a higher permeability for methane compared to other higher hydrocarbons, resulting in a permeate stream of highly pure methane essentially void of higher hydrocarbons.
German Patent Application Publication DE 10 2008 004077A1 (English Translation), to König (MAN Diesel SE), discloses a method and apparatus using membrane separation for the treatment of natural gas for use in a gas engine. An objective of the invention is to improve the fuel gas quality by increasing the methane number, thereby reducing common knocking problems found in poor quality fuel. The fuel to be treated has a methane number of at most 90, especially at most 80, and preferably at most 70.
The problem of upgrading raw gas in the field, such as to sweeten sour gas, is addressed specifically in U.S. Pat. No. 4,370,050, to Fenstermaker. In this patent, Fenstermaker teaches a process that uses a membrane, selective for hydrogen sulfide and/or heavier hydrocarbons over methane, to treat a side stream of raw gas. The process produces a membrane residue stream of quality appropriate for engine fuel. The contaminants pass preferentially through the membrane to form a low-pressure permeate stream, which is returned to the main gas line upstream of the field compressor. Such a process relies on there being sufficient compressor capacity available to handle the return stream that is recycled to the compressor inlet.
However, if the raw gas requires more than a minor adjustment in composition, the proportion of gas that has to be recycled to the compressor may be comparatively large. For example, to upgrade the methane content from 70% to 80%, or from 80% to 90%, may require as much as 50% of the gas being treated by the membrane to be returned for recompression. If the gas is more heavily contaminated, such as containing hydrogen sulfide at the percent level, for example, as is not uncommon, the proportion returned on the low pressure side may be even higher, such as 60% or more. As well as diverting compressor capacity, this makes for an inefficient use of fuel, since fuel gas created by the membrane is used in part to recompress the fuel reject stream.
In commonly owned U.S. Pat. No. 6,053,965 (hereinafter referred to as “the '965 patent”), we disclosed a membrane-based process for conditioning natural gas containing C3+ hydrocarbons and/or acid gas, so that it can be used as combustion fuel to run gas-powered equipment, including compressors, in the gas field or a gas processing plant. The method disclosed in the '965 patent differs from previous membrane-based processes available for field engine fuel conditioning in that it creates substantially lesser quantities of low-pressure gas per unit volume of fuel gins produced. This is achieved by using a membrane separation step in conjunction with a condensation step under pressure, for which the cooling is provided by the membrane separation step, and by balancing the amount of contaminants removed in the condensation and membrane separation steps.
According to the general process disclosed in the '965 patent, a portion of gas from a high-pressure gas stream is withdrawn, then cooled by passing the portion through a heat-exchange step in heat-exchanging relationship against a membrane residue stream. The portion is then separated into a liquid phase comprising C3+ hydrocarbons and a gas phase depleted in C3+ hydrocarbons. The gas phase is then passed across the feed side of a membrane unit containing a membrane selective for C3+ hydrocarbons over methane. A C3+-depleted membrane residue stream is then withdrawn from the feed side and passed back to the heat exchange step. A C3+-enriched permeate stream is withdrawn from the permeate side. The membrane residue stream may optionally be used as combustion fuel for a prime mover.
A schematic drawing of a basic embodiment of the process disclosed in the '965 patent is shown in FIG. 1. Referring to this figure, stream 101, which is the stream to be treated by the process, is withdrawn from high-pressure natural gas stream, 100. Stream 101 passes into heat exchanger, 102, where gas stream 101 is brought into heat-exchanging relationship with C3+-depleted membrane residue stream, 109.
Cooling of residue stream 109 results in the formation of a two-phase mixture, 103, which exits heat exchanger, 102, and passes into phase separator, 104. The liquid phase, containing liquefied hydrocarbons, water, and dissolved gases, is withdrawn as condensate stream, 105.
The high-pressure gas phase from the separator stream, 106, passes to the membrane separation unit, 107, where it is separated into contaminant-enriched permeate stream, 108, and contaminant-depleted residue stream, 109. Stream 108 is withdrawn from the membrane permeate side and may optionally be reintroduced into the main gas flow on the low-pressure inlet side of the pipeline compressor, if any. Depending on the relative volume flow rates of streams 108 and 100, as much as 20-30% or more of the compressor capacity may be used to recompress the gas that is returned to the local pipeline.
Residue stream 109 is withdrawn from the membrane feed side and passes to heat exchanger 102, whence it emerges as conditioned fuel gas stream, 110.
The process of the '965 patent requires the conditioned membrane residue stream to be decompressed before being used as fuel. Decompression results in cooling of the gas to a low temperature, and heating the gas to avoid hydrate formation adds energy-consuming inefficiency to the process.
Thus, although the processes of the '965 patent and the other patents above represent useful improvements over then prior art processes, there remains a need for a process that provides an improved fuel gas for field use, but that is more efficient in terms of compression requirements and overall energy efficiency.